The Copa Libertadores is the South American equivalent of the Champions League
Two Mexican clubs have withdrawn from South America's Copa Libertadores because of the swine flu outbreak that has claimed 42 lives in the country.
Chivas Guadalajara and San Luis' opponents refused to play the away leg of their ties in the usual venues.
Plans to switch the matches to the Colombian city of Bogota were blocked by local authorities there.
Nacional of Uruguay and Sao Paulo of Brazil now look likely to advance to the quarter-finals unopposed.
San Luis were due to host Nacional on 6 May with Guadalajara scheduled to play Sao Paulo the same day, but the South American Football Confederation was forced to postpone the matches.
After Bogota fell through as an alternative, the Chilean Football Association offered to host the matches before their national government also refused permission.
Having considered the possibility of playing just one match to decide the tie, the Mexican Football Federation withdrew both sides from the competition.
The schedule for the tournament, the continent's equivalent of Europe's Champions League, allows little room for manoeuvre.
The second legs of the last 16 matches are due to be played next week and two rounds of World Cup qualification fill the three-week gap between the quarter and semi-finals.
The decision comes a day after Mexican authorities decided to reopen stadiums in the country to spectators after a week of playing behind closed doors.
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